This invention relates to a hydraulic autotensioner for keeping constant the tension in a belt such as a timing belt for driving camshafts of an automotive engine or a belt for driving engine auxiliaries such as an alternator.
A conventional hydraulic autotensioner is disclosed in Unexamined Japanese Utility Model Publication 5-8099. FIG. 6 shows this autotensioner. It has a cylindrical body 30 in which is sealed a hydraulic oil, and a plunger 31 slidably mounted in the body 30. A narrow gap is provided between the body 30 and the plunger 31 through which the hydraulic oil can leak. The plunger 31 has a hole 32 in the top thereof in which is inserted the bottom end of rod 33. The plunger 31 partitions the interior of the body 30 into a pressure chamber 34 and a reservoir chamber 36. The pressure chamber 34 houses a plunger spring 35 which biases the plunger 31 upward to press it against the bottom end of the rod 33. Mounted in the reservoir chamber 36 is a rod spring 37 by which the rod 33 is urged in such a direction as to protrude from the body 30.
The pressure chamber 34 and the reservoir chamber 36 communicate with each other through a return passage 38 formed in the plunger 31 and the rod 33. A check valve 39 is provided at the bottom end of the passage 38. It prevents the flow of hydraulic oil from the pressure chamber 34 to the reservoir chamber 36 through the passage 38.
The check valve 39 comprises a check ball 40 for opening and closing the bottom end of the return passage 38, and a retainer 41 for restricting the movement of the check ball 40.
This autotensioner is mounted with the top end of the rod 33 pressed against a pivotable pulley arm 42. A tension pulley 43 carried on the pulley arm 42 is thus biased in such a direction as to increase the tension in a belt 44.
If the tension in the belt 44 begins to increase in this state, the rod 33 is pushed into the body 30, so that the pressure in the pressure chamber 34 increases. The pressure in the pressure chamber thus dampens the downward movement of the rod 33. When the force urging the rod 33 downward exceeds the pressure in the pressure chamber 34, the hydraulic oil in the pressure chamber 34 begins to leak through the narrow gap 45 formed between the sliding surfaces of the body 30 and the plunger 31, allowing a gradual descent of the plunger 31. When the force urging the rod 33 downward becomes equal to the biasing force of the spring 37, the plunger 31 will stop.
When the belt 44 slackens, the rod 33, biased by the rod spring 37, protrudes quickly from the body, thus eliminating any slackening of the belt 44. As the rod 33 protrudes, the plunger 31 moves upward with the rod, so that the pressure in the pressure chamber 34 begins to decrease, which causes the check ball 40 to move down, opening the return passage 38. Thus, the hydraulic oil in the reservoir chamber 36 is allowed to quickly flow into the pressure chamber 34 through the return passage 38.
Thus, this autotensioner shows a larger resistance to the force urging the rod 33 into the body 30. But when this force disappears, the rod is allowed to protrude quickly, thus instantly eliminating any slackening of the belt. As a result, the tension in the belt 44 is kept constant.
One problem of this autotensioner arises from the structure of the check valve 39. Namely, since this check valve is made up of two separate members, i.e. check ball 40 and retainer 41, it is troublesome to assemble and thus its manufacturing cost tends to be high.
Moreover, since the check ball 40 is moved between the open and closed positions according to the pressure in the pressure chamber 34, it is necessary to limit the moving distance of the check ball 40 to a minimum in order that the check ball 40 can return to its closed position with high reliability. This means that the gap between the check ball 40 and the bottom end of the return passage 38 when the ball 40 is in its open position is correspondingly narrow. Thus, when the valve is in its open position, it is impossible to return the hydraulic oil in the reservoir chamber 36 into the pressure chamber 34 with sufficiently high speed, so that the rod 33 and the plunger 31 can not follow the movement of the belt 44 if the belt slackens abruptly.
An object of this invention is to provide a hydraulic autotensioner which is made up of a smaller number of parts, which can be assembled easily, and whose rod and plunger can quickly follow any sudden slackening of the belt.